血管内超声检查术

        Imagine as a doctor, your patient has a blood clot. Although you know of its existence, the procedure is very delicate. You will essentially be doing this procedure blindly. That’s why you need a device that is even smaller than blood vessels and can help you see, not only the inside but beyond the blood vessels. This system is called the IVUS or intravascular ultrasound: Intravascular meaning within the blood vessels or arteries and ultrasound which is a method of imaging that uses sound. Unlike other imaging devices such as MRIs and X-Rays, IVUS imaging has much better imaging of blood vessels. They can give close-up images to see more details of the blood vessel as well as have better brightness contrast. After sending out sound waves from the tip of the device, it can receive soundwaves that might have bounced back from its surroundings. IVUS systems are like the eye of surgeons because they can show them what is going on in the blood vessel as they are in procedures. This is important because it greatly improves the success of removing blood clots or keeping the blood vessel open with stents. If the blood vessel narrows, it can lead to many complications such as, pulmonary embolism and deep vein thrombosis which can lead to heart attack, stroke, and kidney failure.

        To understand what these IVUS devices show, you firstly need to learn about the blood vessel. They are separated into 3 main parts, from outer to inner sections: adventitia, media, and intima. The plaque, also known as the clot, will be between the media and intima. Due to the material of the plaque and other sections of the blood vessel, it will appear differently on the image. The plaque can be soft, which will appear dark, hard, which will appear bright, or it can be calcified, which, not only is bright, will block imaging for the rest of the blood vessel behind it. In the middle, the dark region is the lumen which is where the actual blood flows through. A media to lumen ratio of 60% is considered significant and needs to be considered for a procedure.

        There are two main types of IVUS, mechanical and phased array IVUS. Mechanical IVUS has a single transducer element, which is what sends and receives ultrasound waves, at the tip of the catheter, which is a long tube that goes in the vessel. These work by rotating the transducer elements using an external motor so it can take a 360-degree image around it. Phased array IVUS systems do not need to rotate because they just have transducer elements all around the tip of the catheter. They separately activate in a rotating fashion and also create an image.

        There are three main ways to assess an image produced by an IVUS device. These are the resolution of the image, how much it can show, and its frames per second it can show. Because of the nature of ultrasound, the more the soundwaves penetrate to help you see deeper, the worse resolution it will get. This is because you need larger soundwaves to penetrate deeper, but in turn, can not show details well because of its size. On average, mechanical IVUS use 40-45 MHz frequencies while phased arrays use 20 MHz. This means mechanical IVUS resolution is better compared to phased arrays. Current IVUS systems can produce images at 30 frames per second. Mechanical systems’ frames per second are dependent on the rotational speed, the pulser/reciever (PR) to support high pulse repetition frequency, and the analog to digital converter (ADC) to handle sample data at high speeds.

        IVUS imaging is amazing because of its ability to show doctors the blood vessels they are working on as it happens. It can show all parts of the blood vessel like an x-ray to give doctors as much information as possible. This is all crucial in making sure major problems like heart attacks and strokes can be dealt with before they happen to make everyone safer.